1. Field of the Invention
This invention is in the field of introducing fluorescent reagents into or applying fluorescent reagents to cells and monitoring the fluorescence in the cells.
2. Description of the Prior Art
Drug discovery is a long, multiple step process involving the identification of specific disease targets, development of an assay based on a specific target, validation of the assay, optimization and automation of the assay to produce a screen, high throughput screening of compound libraries using the assay, hit validation and hit compound optimization. The output of this process is a lead compound that goes into preclinical and eventually clinical trials. In this process, the screening phase is distinct from the assay development phases and testing the efficacy of the compounds in living biological systems.
Performing a screen on many thousands of compounds requires parallel handling and processing of many compounds and assay component reagents. Standard high throughput screens use homogeneous mixtures of compounds and biological reagents along with some indicator compound loaded into arrays of wells in standard microtiter plates with 96 or 384 wells. The signal measured from each well, either fluorescence emission, optical density, or radioactivity, integrates the signal from all the material in the well giving an overall population average of all the molecules in the well. This type of assay is commonly referred to as a homogeneous assay.
Science Applications International Corporation (SAIC) 130 Fifth Avenue, Seattle, Wash. 98109 describes an imaging plate reader. This system uses a CCD camera to image the whole area of a 96 well plate. The image is analyzed to calculate the total fluorescence per well for homogeneous assays.
Molecular Devices, Inc. describes a system (FLIPR) which uses low angle laser scanning illumination and a mask to selectively excite fluorescence within approximately 200 microns of the bottoms of the wells in standard 96 well plates in order to reduce background when imaging cell monolayers. This system uses a CCD camera to image the whole area of the plate bottom. Although this system measures signals originating from a cell monolayer at the bottom of the well, the signal measured is averaged over the area of the well and is therefore still considered a homogeneous measurement, since it is an average response of a population of cells. The image is analyzed to calculate the total fluorescence per well for cell-based homogeneous assays.
Proffitt et. al. Cytometry 24: 204–213 (1996) describes a semiautomated fluorescence digital imaging system for quantifying relative cell numbers in situ in a variety of tissue culture plate formats, especially 96-well microtiter plates. The system consists of an epifluorescence inverted microscope with a motorized stage, video camera, image intensifier, and a microcomputer with a PC-Vision digitizer. Turbo Pascal software controls the stage and scans the plate taking multiple images per well. The software calculates total fluorescence per well, provides for daily calibration, and configures easily for a variety of tissue culture plate formats. Thresholding of digital images and reagents which only fluoresce when taken up by living cells are used to reduce background fluorescence without removing excess fluorescent reagent. For example, in this system cells are pretreated with fluorescein diacetate (FDA) and loaded into 96-well plates.
Fluorescence microscopy of cells and tissues is well known in the art. A variety cf methods have been developed to image fluorescent cells in a microscope and extract information about the spatial distribution and temporal changes occurring in these cells. An article by Taylor, et al. in American Scientist 80 (1992), p. 322–335 describes many of these methods and their applications. These methods have been designed and optimized for the preparation of a few specimens for high spatial and temporal resolution imaging measurements of distribution, amount and biochemical environment of the fluorescent reporter molecules in the cells.
Treating cells with dyes and fluorescent reagents and imaging the cells is well known in the art. There is also a considerable body of prior art related to genetic engineering of cells to produce fluorescent proteins, such as modified green fluorescent protein (GFP) as a reporter molecule. The green fluorescent protein (GFP) of the jellyfish Aequorea victoria is a protein with an excitation maximum at 395 nm and an emission maximum at 510 nm and does not require an exogenous factor. Uses of GFP for the study of gene expression and protein localization are discussed in more detail in papers by Chalfie et al. in Science 263 (1994), p. 12501–12504. Some properties of wild-type GFP are disclosed by Morise et al. in Biochemistry 13 (1974), p. 2656–2662, and Ward et al. in Photochem. Photobiol. 31 (1980), p. 611–615. An article by Rizzuto et al. in Curr. Biology 5 (1995), p. 635–642 discusses the use of wild-type GFP as a tool for visualizing subcellular organelles in cells. A paper by Kaether and Gerdes in Febs Letters 369 (1995), p. 267–271, reports the visualization of protein transport along the secretory pathway using wild-type GFP. The expression of GFP in plant cells is discussed by Hu and Cheng in Febs Letters 369 (1995), p. 331–334, while GFP expression in Drosophila embryos is described by Davis et al. in Dev. Biology 170 (1995), p. 726–729. U.S. Pat. No. 5,491,084 describes expressing GFP from Aequorea victoria in cells as a reporter molecule fused to another protein of interest. PCT/DK 96/00052 relates to methods of detecting biologically active substances affecting intracellular processes by utilizing a GFP construct having a protein kinase activation site. Numerous references are related to GFP proteins in biological systems. For example, PCT/US94/10165 describes a system for isolating cells of interest utilizing the expression of a GFP like protein. PCT/GB96/00481 describes the expression of GFP in plants. PCT/US95/01425 describes modified GFP protein expressed in transformed organisms to detect mutagenesis. U.S. Pat. Nos. 5,401,629 and 5,436,128 describe assays and compositions for detecting and evaluating the intracellular transduction of an extracellular signal. Recombinant cells that express cell surface receptors and contain reporter gene constructs that include transcriptional regulatory elements that are responsive to the activity of cell surface receptors are used.